A bridge too far – weeds a reservoir for hidden diseases
Weeds that survive in a bridge, both living and dead, harbour multiple crop diseases including crown rot, rust and powdery mildew, says Sue Thompson from the Queensland Department of Agriculture, Fisheries and Forestry (DAFF).
“Crop stubble has been aiding survival of these diseases, and we’ve seen stubble-borne pathogens increasing for many years. But we haven’t considered weeds in the same light,” Ms Thompson says.
Research is showing that diseases such as wheat crown rot and rusts use several grass hosts as green bridges. Sunflower powdery mildew and rust live in wild sunflower populations and the damaging sunflower pathogen Phomopsis gulyae, which causes stem cankers on a range of crops, survives on Noogoora burr.
“We are finding that a number of diseases, such as Phomopsis and Fusarium species, can still survive even after the weeds have been sprayed with herbicide, so it is important to also bury dead weeds so they break down.”
Green bridges provide a between-season host for diseases, Ms Thompson says.
“If high levels of pathogens such as Phomopsis or the crown rot Fusarium pathogens are present in a green bridge when crops are sown, even crops selected for their disease tolerance can be affected if weather conditions are favourable. That's why it is important to control green bridges before diseases infect the weeds within them.”
Spraying the green weed bridge is considered to be the most effective option for controlling harboured diseases. The aim of spraying is to achieve an outright kill of weeds and volunteers. Glyphosate and other translocated herbicides are most effective.
Heavy grazing can be an effective technique for reducing large masses of summer vegetation. However, because host plants are not killed, they can continue to harbour diseases, particularly root and crown diseases.
Tillage is also an effective option; however, growers need to consider the associated risks of erosion and loss of soil moisture.
Selective-tillage brown bridge refers to the role of dead weeds and stubble in helping pests and diseases cross from one cropping season to the next.
“Just because you spray out the weeds and kill them doesn’t mean the disease problem goes away,” Ms Thompson says.
Some diseases such as Fusarium and Phomopsis stem canker continue to survive on dead plant tissue, remaining dormant. When growers plant another susceptible crop in that paddock, the crop is at risk of being infected.
“To overcome these risks, growers may need to consider strategic tillage, particularly after a serious outbreak of stubble-borne or weed-borne disease.”
Paul McIntosh, senior agronomist for Landmark, has been assisting Ms Thomson with her research for the past year.
“I’d never heard the term ‘brown bridge’ until Sue explained it to me a year ago. Sue asked me to collect plants like Noogoora burr for her, which I did from Glenmorgan through to Kingsthorpe in Queensland,” Mr McIntosh says.
Mr McIntosh, who has been working as a crop adviser in the northern region for 35 years, says: “I didn’t think much of it until Sue explained to me the results and possible ramifications.”
“What I’ve taken away from Sue’s research is that we need to be proactive in controlling all our weeds, both broadleaf and grass types. To do this, there are real benefits in using more residual-type pre-emergent herbicides such as imazapic in our fallow to prevent weed germination and growth.”
Mr McIntosh says it also makes sense to incorporate strategic tillage into the way growers control both green and brown bridges.
”We’ve been performing strategic tillage for a few years now to help control herbicide-resistant plants in our green bridges and stubble. Sue’s research provides more evidence as to its benefits; in this case, to control diseases that are potentially hosted in these dead weeds.”
A community issue
Controlling weeds in green and brown bridges is a community issue and requires neighbours to work together to remove volunteers and weeds, Ms Thompson says.
“It’s an uphill battle for growers working towards limiting pathogen survival in paddocks if all the neighbouring channels, fencelines, roads and paddocks are loaded with infected weeds.
“Water, wind and mechanical transfer of inoculum from diseased weed reservoirs can rapidly spread disease so it’s important for neighbours to talk to each other and plan together on how they will manage these areas.”
Ms Thompson also encourages growers to keep themselves informed of the issue: “We encourage growers and advisers to attend workshops to learn how to identify and manage for diseases of their rotational crop selections and also their multiple weed hosts. At these workshops we try to emphasise how important it is to consider disease risk in terms of both current crops and future planned rotations.”
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